FOUS30 KWBC 210812
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Mon Jul 21 2025

Day 1
Valid 12Z Mon Jul 21 2025 - 12Z Tue Jul 22 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF 
KENTUCKY, TENNESSEE, AND PORTIONS OF THE SOUTHERN APPALACHIANS...

...Southern Ohio Valley into the Southern Appalachians...

Evolving pattern with a seasonally strong surface ridge to the
north will aid in pressing the persistent quasi-stationary boundary
across the Mid Atlantic and Ohio Valleys to the south leading to a
degrading precip field north of the front, but maintaining
convective posture along and south of the boundary. A panoply of
weak shortwave perturbations will be rippling through southern Ohio
Valley into the southern Mid Atlantic on Monday with expected
diurnal destabilization over much of KY/TN into southwestern VA
leading to another period of scattered to widespread convective
flare ups in the prevailing warm sector. A relatively buoyant
environment in place across the above areas will promote locally
heavy downpours capable of rates >2"/hr, something that has been
customary within this persistently moist airmass lingering from the
western fringes of the Western Atlantic ridge. Expect PWATs to hang
within the +1 to +2 standard deviation range across the above areas
with the latest 00z HREF mean MUCAPE to hover between 1000-2000
J/kg for much of the Southern Appalachians with 2000-3000 J/kg
presence across much of KY and TN. The elevated CAPE indices
coupled with the moisture rich environment of >2" PWATs will lead
to scattered instances of flash flooding, especially over those
terrain focused areas of eastern KY/TN into southwest VA where
multiple instances of heavy rainfall have led to a series of FFW's
being issued the past several days. 

00z HREF neighborhood probs remain high (>60%) for at least 1" of
rainfall with >2" running between 40-60% over a large area
encompassed by low FFG indices for 1/3/6hr increments. The
positioning of the front will be factor for the northern periphery
of the greatest convective reflection in the setup, as well as a
focus for anchored cores that could relish on the capability to
drop between 2-4" of rainfall in spots as depicted within the
latest 00z CAMs suite. Considering both the probabilistic and
deterministic maxima being depicted within the hi-res suite, there
was enough consensus to maintain general continuity of the previous
SLGT risk inheritance with a southward shift in the risk area
overall to reflect the latest trends of frontal positioning and
convective output within the CAMs. 

...Southern Mid Atlantic into the Southeast...

Scattered strong convective cores will be capable of heavy rainfall
in pretty much any area within the bounds of southern VA down
through the Carolina's into the Southeastern CONUS. The best
threats for more organized heavy convective regimes will be located
over eastern NC and the north-central FL Peninsula. The two
patterns will not be related in any way, but they do share an
element of robust instability and a foci for convection along a
surface based front/boundary in the form of a cold front (NC) and
surface trough (FL). FFG's within each area are still pretty high
overall for all 1/3/6hr indices, so the threat for more widespread
flash flood concerns is lower, but still worthy of a risk
considering the environmental factors in play. Elevated probs for
>3" locally (50-70%) are sufficient for those flash flood concerns
for each area respectively. However, a sharp decline in the
prospects for >5" really mutes the higher risk potential and keeps
the threat within the MRGL threshold. As a result, there was very
little deviation from the previous forecast with only minor
adjustments over the Southern Mid Atlantic coast near the VA
Tidewater. MRGL risk remains in effect. 

...Southwest U.S...

The southwestern U.S will maintain a persistent convective posture
with a defined monsoonal axis across NM/AZ with a sharp cutoff in
deeper moisture as you move closer to the Colorado river Basin.
Isolated stronger cores will lead to threats of flash flooding just
about anywhere over the two respective areas with emphasis on those
places dealing with remnant burn scars and complex terrain which
exacerbate flash flooding impacts. Strongest signal for heavy rain
in the prob fields remains parked over southeast AZ with the
Huachuca Mtns. the likely beneficiary of slow-moving thunderstorms
and QPF maxima >1" during time frame of impact (18-03z). A pretty
stable element this time of year with a generally seasonal pattern
in place. This is more than enough to constitute a MRGL risk
issuance given the setup.

...Northern Plains through Upper Midwest...

Two-part convective episode over the northern tier of the CONUS 
will lead to a few areas seeing repeated heavy rainfall prospects 
in the D1 time frame. The first of which will be the migration of a
complex out of the Dakotas into the Red River/Bois de Sioux basins
that border ND/SD/MN in the early portion of the period with
locally heavy rainfall in wake of the passing complex. Still some
discrepancy on the eastward extent and residual QPF magnitude that
will be produced by the complex, but the pattern generally favors a
weakening component over the area above, so perhaps the threat is
on the lower end for flash flood concerns in the first 6hrs of the forecast.

The second time frame of interest will arise over the course of the
following evening as an approaching trough axis across the PAC
Northwest will lead to an increasingly diffluent pattern downstream
of the mean trough, promoting favorable large scale ascent from
eastern MT up into the Dakotas. A series of smaller shortwaves will
propagate northeast into the region after 00z with convective
initiation most likely further west with more question marks on the
maintenance of convection as you step eastward. There's a mixture
of results within the CAMs output for whether the pattern evolves
in such that eastern ND into MN gets put into play for heavy
rainfall overnight Monday into early Tuesday, or is a majority of
the pattern only aligned within eastern MT into ND. The split in
guidance has relegated the signal within the EAS to a paltry <10%
for >1" in the 6hr period between 00-06z Tuesday, but we do see an
uptick in the 06-12z Tuesday window, so perhaps it could be more of
a timing of the convective pattern moving downstream with less
focus on discrete activity prior to the advancing convective
complex. The general rule of thumb for these types of patterns is
favoring a lower risk until there's a greater consensus on what
could transpire. This works well within the area of interest due
to modest FFG indices in place and FFG exceedance probs remaining
on the lower end. A broad MRGL risk is in place for the
aforementioned areas with the greatest concerns across the eastern
Dakotas and eastern Montana.  

...Pacific Northwest...

Combination of a weak upper low presence over southwest Canada and
the approach of a more robust shortwave trough into the
Northwestern CONUS will lead to focused areas for isolated flooding
concerns within the terrain of northeast OR and the Washington
Cascades. Signals for locally >1" of rainfall are >70% in each
respective locale, but signals in the deterministic are still
spotty in the grand scheme. The pattern is just conducive for these
localized heavy rain prospects in complex terrain and burn scar
remnants located over northern WA state. Weak IVT advection into
the region along with favored large scale ascent downstream of the
approaching trough should lead to periods of convection during the
height of the convective time frame (18-06z). The signal remains
low-end, but enough to warrant two small MRGL risks over the
aforementioned areas. 

Kleebauer
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